When optical lines are accommodated by using wavelength division multiplexing (WDM), an operation management system allocates wavelengths to the respective optical lines. A process for allocating wavelengths to optical lines to be allocated is referred to as wavelength allocation design. Note that an “optical line” in the description below corresponds to an optical path (or a wavelength path) that is established between two nodes.
In the wavelength allocation design, wavelengths are allocated to respective optical lines that are established on each optical fiber in such a way that the wavelengths of the respective optical lines are different from each other. In addition, in order to suppress the cost of a network, the same wavelength is used on a route from a start point node to an end point node in many cases.
Conventionally, when an optical line is established in order to provide a certain communication service, the optical line is not deleted until the service is terminated. However, in recent years, an operation form in which an optical line of a needed bandwidth is established as needed has been widely used due to, for example, software defined networking (SDN). Namely, an optical line may be frequently added or deleted. Therefore, even in a case in which wavelengths are allocated to respective optical lines in such away that wavelength usage efficiency is high at a particular point in time, when an optical line is added or deleted afterward, the wavelength usage efficiency may decrease. Accordingly, an operation is requested in which the wavelength usage efficiency is monitored, and when the wavelength usage efficiency decreases, wavelength allocation to the respective optical lines is changed. Note that an action to change wavelength allocation during operation may be referred to as wavelength reallocation or defragmentation (or simply referred to as “defrag”).
As a related technology, a wavelength path reallocation method for designing wavelength paths in such a way that a used frequency area becomes smaller than before reallocation has been proposed (for example, US Patent Publication No. 2013/0195460).
An optical signal modulation scheme is selected according to transmission distance, requested quality, or the like. In a WDM optical network, optical signals of different modulation schemes may be multiplexed. In the example illustrated in FIG. 1A, the modulation schemes of optical signal 1 to optical signal 4 that are multiplexed into a WDM optical signal are different from each other. Here, a bandwidth needed to transmit each of the optical signals (hereinafter, this may be referred to as a “modulation bandwidth” or may be simply referred to as a “bandwidth”) depends on a modulation scheme. In addition, in order to improve wavelength usage efficiency, the variable wavelength spacing scheme in which optical signals can be arranged at a desired wavelength spacing has been proposed, as illustrated in FIG. 1B. Note that the wavelength usage efficiency can be improved in the variable wavelength spacing scheme in comparison with the fixed wavelength spacing scheme illustrated in FIG. 1A. Further, the network topology of a long-distance transmission network is simple, but many nodes are connected complicatedly in a metropolitan area network. Namely, the network topology of the metropolitan area network is complicated.
As described above, in a WDM optical network in which network topology is complicated and modulation bandwidths allocated to respective optical lines are not constant, it is difficult to determine whether the wavelength usage efficiency is high or low. Namely, it is difficult to determine whether it is preferable that wavelength reallocation be performed in the WDM optical network.